In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically below 100 mPas at 25° C. although in most applications the viscosity should be below 50 mPas, and often below 25 mPas. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature which is often elevated to about 40° C. (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent.
In one common type of inkjet ink this liquid is water—see for example the paper by Henry R. Kang in the Journal of Imaging Science, 35(3), pp. 179-188 (1991). In those systems, great effort must be made to ensure the inks do not dry in the head due to water evaporation. In another common type the liquid is a low-boiling solvent or mixture of solvents—see, for example, EP 0 314 403 and EP 0 424 714. Unfortunately, inkjet inks that include a large proportion of water or solvent cannot be handled after printing until the inks have dried, either by evaporation of the solvent or its absorption into the substrate. This drying process is often slow and in many cases (for example, when printing on to a heat-sensitive substrate such as paper) cannot be accelerated.
Another type of inkjet ink contains unsaturated organic compounds, termed monomers, which polymerise by irradiation, commonly with ultraviolet light, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures. In such inkjet inks it is necessary to use monomers possessing a low viscosity.
Inkjet inks based on curable components can contain monofunctional and multifunctional components. The monofunctional components are typically soft, flexible and provide good adhesion properties while multifunctional components typically provide good cure and surface hardness. A combination of monofunctional and multifunctional components is often needed to provide a good balance of properties. However, the use of monofunctional components in an ink often leads to problems. More specifically, the use of monofunctional monomers, and in particular, cyclic monofunctional (meth)acrylate monomers such as PEA and CTFA, often leads to problems in the film after exposure to radiation because some uncured monomers remain in the film after exposure to radiation. One such problem associated with an incomplete cure of the film after exposure to radiation is odour issues. This is particularly a problem for inks which comprise cyclic monofunctional (meth)acrylate monomers, such as PEA and CTFA, which are necessary to provide advantageous properties to the ink but often remain uncured in the film after exposure to radiation. Such uncured monomers a free to move in the film which causes problems, such as the release of an undesirable odour. This is less of a problem in inks comprising multifunctional monomers as most multifunctional monomers crosslink or at least partially crosslink on exposure to radiation owing to an increased number of functional groups present in the monomer which can react. This means that the crosslinked multifunctional monomers are bound within or to the film, meaning that the film is more fully cured and hence do not suffer the problems associated with an incomplete cure of the film. This is in contrast to monofunctional monomers, which often remain uncrosslinked in the film after exposure to radiation, meaning that they are present as a liquid in the film and result in problems associated with an incomplete cure, such as an undesirable odour in the film. Monofunctional monomers are however necessary to provide other desirable properties. In this regard, an ink which comprises only multifunctional monomers is often brittle.
In order to prevent such issues with inks comprising monofunctional monomers and particularly cyclic monofunctional (meth)acrylate monomers, photoinitiator packages have been designed which often include many photoinitiators in the ink, for example, a five component photoinitiator package is often needed to ensure that more of the monofunctional monomers are cured and hence reduce the problems associated with an incomplete cure, such as odour problems. Such multi-photoinitiator systems are not desirable owing to the complexity and expense of such systems. Furthermore, photoinitiators can also release unwanted odours.
There is therefore a need in the art for an inkjet ink that comprises a cyclic monofunctional (meth)acrylate monomer but reduces the problems associated with an incomplete cure after exposure to radiation, such as odour issues after curing of the ink, maintains the advantageous properties of an inkjet ink comprising a cyclic monofunctional (meth)acrylate monomer and can achieve such advantages using fewer photoinitiators.